The present invention relates to a method of controlling synchronous motors and more particularly, to a position-sensorless controlling method of synchronous motor, that is, a controlling method of synchronous motor without resort to position sensor.
The structure of a rotor of a synchronous motor is classified into a salient type structure in which the winding inductance changes with the rotation position, and a non-salient type structure in which the winding inductance is substantially constant. A permanent magnet constituting the rotor is embedded in a core in the former structure, and it is arranged on the surface of the rotor in the latter structure.
There are two kinds of methods for driving the synchronous motor. Namely,
(1) A so-called synchronous operation method in which the rotation position is not detected, and the synchronous motor is driven in open loop like the inverter control for induction motors.
(2) A so-called brushless DC motor operation method in which any rotary position detecting means is used to operate the synchronous motor in closed loop.
Of these methods, in the synchronous operation method, it is required that an optimum motor voltage conforming to a load be applied in accordance with an inverter frequency. Accordingly, unless any optimum voltage control is carried out, not only an increase in motor current will be caused but also a shortage of torque will occur during an abrupt load change operation or an abrupt accelerating/decelerating operation, thus causing the motor to stop.
On the other hand, in the brushless DC motor operation, any rotor position sensor is provided and as compared to the inverter drive for induction motors, the number of wiring lines between an inverter and the motor increases to degrade the maintainability and reliability, thereby preventing a widespread use of this method for general industries and its use in special ambience such as a compressor. To eliminate the above disadvantages, various kinds of position-sensorless techniques for detecting the position without using a rotor position sensor have been announced.
To sum up the position-sensorless techniques, there are available a position-sensorless technique adapted for stop/low-speed region and a position-sensorless technique adapted for medium/high-speed region. The stop/low-speed position detection is applied to the salient motor and takes the advantage of the fact that the winding inductance differs with rotation positions to measure the position through such an expedient as injection of a high-frequency signal.
In the medium/high-speed region, a voltage induced in the motor winding is utilized and various kinds of utilization methods have been announced.
For example, one may refer to a technique disclosed in JP-A-8-308286. In this technique, in relation to a d-q real (actual) rotary coordinate system having d-axis representing positions in the flux direction of the permanent magnet rotor and q-axis representing positions 90xc2x0 leading the d-axis in the rotation direction, a dc-qc control rotary coordinate system having dc axis representing virtual rotary positions from the control viewpoint, and qc-axis representing virtual positions 90xc2x0 leading the dc-axis in the rotation direction is defined. On the d-q real rotary coordinates, a motor model is expressed in accordance with an equation indicative of the relation between current and voltage by using motor parameters such as motor resistance, motor inductance and motor generation constant, and it is demonstrated that a difference between d-axis current predicted from the motor model and dc-axis current on the control axis is proportional to position error xcex94xcex8.
In predictive calculation of the d-axis current, on the assumption that the motor parameters are the same for the motor model and the real motor and xcex94xcex8 is close to zero, approximation of sin xcex94xcex8=xcex94xcex8 is carried out. Further, currents on the dc-qc rotary coordinate axis observed from the control viewpoint are used as current values used for calculation.
As another method, a technique in JP-A-9-191698 is known. In this technique, a motor induced voltage generated concurrently with rotation of the motor, as viewed from the stop state, is handled as external disturbance, and pursuant to the well-known external disturbance observer method, the magnitude and polarity of the motor induced voltage are estimated. Like the previously described prior art, the external disturbance observer is based on the state equation in the dc-qc rotary control coordinate system. Next, a speed is computed by using the estimated motor induced voltage and motor parameters and besides, the speed is integrated to provide position information so that a shift from the real rotation position may be corrected using position error xcex94xcex8 obtained from a dc-axis component estimation value of induced voltage and the estimated speed.
In the calculation of xcex94xcex8, the approximation of sin xcex94xcex8=xcex94xcex8 is performed on the assumption that xcex94xcex8 is close to zero as in the case of the previous example.
The conventional position-sensorless technique for medium/high-speed region utilizing the induced voltage, however, uses current values on the control rotary coordinate system representing the virtual rotary axis in order to estimate the motor position, and the approximation of sin xcex94xcex8=xcex94xcfx86 is carried out. For these reasons, the accuracy of control is degraded when the positional error is large. Accordingly, the prior art is unsuited for applications in which the load changes abruptly or the operation is accelerated/decelerated abruptly. In addition, when the motor parameters are involved, the estimated position becomes erroneous.
An object of the present invention is to provide, as a position-sensorless technique for medium/high-speed region, a control method suitable for driving a synchronous motor without resort to position sensor.
To accomplish the above object, the present inventor has studied the following points.
(1) Determining axis error xcex94xcex8 by expressing the phase angle by a value not depending on the control axis;
(2) Determining a frequency on the basis of the axis error;
(3) Estimating a speed on the basis of current and voltage not depending on the control axis;
(4) Eliminating the influence of a parameter error contained in the axis error; and
(5) Combining the operation method for stop/low-speed operation with the position-sensorless technique for medium/high-speed region.
In a position-sensorless control method of synchronous motor according to the invention made on the basis of the studies as above, a first phase difference between a motor current of a synchronous motor having a field system of a permanent magnet and a real rotary position, and a second phase difference between the motor current and a virtual rotary position are determined. From the difference between the first and second phase differences, a phase error between the real rotary position and the virtual rotary position is estimated. Since the first and second phase differences are used, the accuracy of control can be high even when the phase error is large.